This invention relates to a signal defect compensator which may be used in a system for reproducing recorded television image information.
In the reproduction of television video signals from information recorded, for example, on magnetic tape, magnetic discs or other types of recording media, a decrease or loss of the recorded signal information may be encountered, which is apparent to the viewer as a degraded picture. A principle form of such a defect is known as a dropout which may occur due to imperfections in the recording media or an accumulation of dirt or other debris at the junction where the playback/recording heads and the media meet. When such a dropout occurs, the signal recovered from the media generally suffers a sharp reduction in amplitude which appears in the picture seen by the viewer as random black and white streaks or flashes on the screen of the television receiver or other means utilized as a reproducer. In severe cases, such a dropout may occur for one or more entire lines of the television picture.
Since it is known that television image information is to some extent redundant from line to line, dropout compensators (DOC) have generally operated on the principle that since the image information is redundant from line to line, it is possible to compensate for a dropout by substituting information from a previous image line during the duration of the dropout. Known defect compensators, as shown in U.S. Pat. No. 2,996,576, therefore, store image information from a previous line and insert the stored previous line information into the output video signal when a signal dropout occurs. Such a direct replacement is generally satisfactory for a monochrome video signal; however, in the case of a color video signal, a direct replacement from the previous line is not satisfactory since the phase relationship between scanning sync and color sync (burst in the NTSC system) differs on adjacent lines of a television picture. The scanning sync pulses are in-phase from line to line, but the color sync (burst) is interlaced or 180.degree. out-of-phase from line to line. If a direct replacement signal from the previous line storage is attempted, the color signals will be inverted, and the replacement appears on the reproduced picture as complementary to the true colors. This affect is particularly disturbing to the viewer if the dropout exists for any appreciable portion of a television line. Therefore, in a color dropout compensating circuit, as shown in U.S. Pat. No. 3,463,874, the stored image information is separated by a low pass filter into a luminance portion containing substantially all the frequencies below 2.5 MHz and by a bandpass filter into a chrominance portion containing substantially all the frequencies from 2 to 4 MHz. The chrominance portion is passed through an inverting signal processor and then recombined with the luminance portion so that the color phasing is correct when the stored video is utilized as replacement video during a dropout.